Microsoft has announced officially that it and Symantec have worked together to take down the dangerous Bamital botnet. This botnet was used to hijack search results on computers around the world, taking the computer user to potentially dangerous websites. Once the users were deposited at the dangerous websites, Bamital operators were able to install malware onto the user's computer to steal personal information or charge businesses fraudulently for online advertisement clicks.

Microsoft’s research shows that Bamital hijacked more than 8 million computers over the past two years. Microsoft says that the botnet affected many major search engines and browsers including Bing, Yahoo, and Google offerings. Microsoft says that it and Symantec took action against the botnet to help protect computer users and promote cloud security for everyone.

Users who are already infected by malware installed by the botnet are being proactively informed that their computers are infected via an official webpage. That official webpage offers the users an easy-to-use method for removing the infection.

The takedown operation was known as Operation b58 and is the sixth botnet takedown operation Microsoft has been part of in the last three years. In September of 2012, Microsoft participated in the takedown of the Nitol botnet.

Absolutley, haven't heard of any other companies contributing to this effort quite as much as they do (though I do not work in the specific field and maybe do not follow the topic close enough to authoritively say that's a true statement).

This seems like it is a pretty costly process, so among all the other free services and apps they offer this one seems pretty big and often gets overlooked.